There are many portable devices with two loudspeakers on the market, such as iPod docks or laptops. Tablets and mobile phones with built-in stereo loudspeakers can be viewed as stereo portable devices. Compared to a conventional stereo system with two discrete loudspeakers, the two loudspeakers of a portable stereo device are located very close to each other. Due to the size of the device, they are usually spaced by only few centimeters, between 10 and 30 cm for mobile devices such as smartphones or tablets. This results in music reproduction which is narrow, almost “mono-like”.
The concept of Mid/Side loudspeaker has been introduced in (Heegaard, F. D. (1992). “The Reproduction of Sound in Auditory Perspective and a Compatible System of Stereophony”, J. Audio Eng. Soc., 40(10), pp. 802-808). The goal was to reproduce a stereo signal with only a single loudspeaker box. As opposed to playing back left and right signals, sum signal, i.e. left signal plus right signal and difference signal, i.e. left signal minus right signal are reproduced with two loudspeakers with different characteristics. The sum signal is played back with a conventional loudspeaker which is omnidirectional at low frequencies and unidirectional at high frequencies. The difference signal is reproduced with a dipole loudspeaker, bi-directionally pointing towards left and right directions. Perceptually, this results in that a listener hears the sum signal (soloists, main content) from the loudspeaker position. Additionally, there is a spatial effect. The dipole, driven with the difference signal, excites the room with zero sound propagation towards the listener.
In the patent application PCT/CN2011/079806, a method for generating an acoustic signal with enhanced spatial effect is described. This method uses the same principle of dipole rendering, applied with normal loudspeaker systems. The original stereo signal is played out on the two loudspeakers and the difference signal is played out with a dipole rendering from the same loudspeaker system, i.e. direct rendering on one side, and multiplied by −1 on the other side. Such a system, however, requires that the listener is in a central listening position. If the listener is not exactly located in front of the loudspeaker system, his sound impression exhibits a sustained decline.